1. Field of the Invention
The present invention relates to a method of controlling a welding current in a direct-current resistance welding machine, and more particularly to a method of controlling the waveform of a welding current depending on the manner in which expulsions are produced or the number of successive welding cycles when a plurality of welding points are successively welded with a welding current having a staircase waveform.
2. Description of the Related Art:
Direct-current resistance welding machines are widely used in the welding industry. According to the welding process using direct-current resistance welding machines, it is customary to control the welding quality by controlling the pressure under which a workpiece is gripped by a gun arm, the time during which a welding current is supplied to the workpiece, and the waveform of the welding current. It is known that the welding current which is controlled has an optimum value lower than and as close to a limit current value (hereinafter referred to as an "expulsion-producing limit current value") beyond which expulsions would be produced as possible, and when a workpiece is welded with such an optimum welding current, no expulsions are generated and a sufficient welding strength, i.e., a tensile shear strength, is achieved.
To weld a workpiece according to such a practice, the operator of a direct-current resistance welding machine detects an expulsion-producing limit current value by gradually increasing or reducing the welding current, determines an optimum welding current value based on the detected expulsion-producing limit current value, and welds the workpiece with the determined optimum welding current value.
A technical concept for controlling the waveform of a welding current is disclosed in Japanese patent publication No. 58-43192 entitled "spot-welding process". According to the disclosed spot-welding process, a workpiece is welded for stable welding strength with a staircase welding current whose value is initially higher than an expulsion-producing limit current value and then lower than the expulsion-producing limit current value after a nugget is formed and the voltage applied between electrodes becomes lower than its peak value.
When a plurality of welding points on the workpiece are successively welded with such an optimum welding current value that is determined according to the disclosed spot- welding process, however, since the expulsion-producing limit current value is gradually increased as the tip ends of the electrodes that grip the workpiece are worn, the welding current will become insufficient and no sufficient welding strength will be achieved if the initially established optimum welding current is maintained throughout the welding operation. Consequently, the disclosed spot-welding process is not effective in successively spot-welding the workpiece.